The News

In the news this month:

Stars come in many sizes, some are small and incredibly dense (a teaspoon of material weighing more than a fully-laden truck), others are huge but rarefied (large enough to swallow the orbit of Jupiter but much less dense than the Sun). Observations suggested that the upper limit on a star's mass was 150 times that of our own Sun and, while recent observations have detected a few stars approaching this mass, the question remains whether this apparent limit is physical (they cannot form larger than this) or statistical (they can, but are so rare that it is unlikely there are any close enough to us to be detectable). Likely locations for such stars would be in high mass, young clusters, since the more massive a star, the faster it uses up its fuel and dies. Part of the problem is that the central parts of massive star forming regions, the very places these stars might be if they form at all, are very crowded places, making imaging and spectroscopy of individual objects a difficult undertaking. But in a paper published in the Monthly Notices of the Royal Astronomical Society during July, a team of astronomers has now found evidence of some record-breaking stars with masses well above the 150 solar mass limit. The team, led by Paul Crowther at the University of Sheffield, used observations from the archives of both the Hubble Space Telescope and the Very Large Telescope in Chile to study stars in two clusters where rapid star formation is still occurring. Different atoms and molecules absorb and emit light at characteristic wavelengths, resulting in a series of absorption or emission lines in a spectrum which can be used to determine what chemicals are present. Since different chemicals are stable within different temperature ranges, the chemical signatures present in a spectrum can give an estimate of the temperature of a star's atmosphere.

The width of these lines is determined partly by the speed of the gas creating them, which is determined by the temperature - the hotter a gas, the faster the particles within it move. These diagnostics give an estimate of the luminosity of a star, from which its mass can be estimated. By examining spectra from NGC3603, an open cluster located in the constellation of Carina, and R136 in the Large Magellanic Cloud, the astronomers estimated the masses of several massive stars and found several that are larger than the theoretical 150 solar mass limit. As stars age they lose material in a stellar wind. Generally, the more massive the star, the stronger the wind and the more material is lost per second. The largest of the stars examined in this survey, R136a1, has a wind speed of roughly two thousand six hundred kilometres per second and a current mass two hundred and sixty five times that of the Sun, but has probably lost an estimated 20% of its mass in the first 1.5 million years of its life so that it may have started out as massive as 320 solar masses, way above the current limit generally used in modelling stellar populations. These findings have implications for our understanding of stellar evolution. Exactly how such massive stars form is not certain, and the question remains as to what happens when they reach the end of their lives. Stars between 8 and 150 times the mass of the Sun end their lives as core collapse supernovae, leaving behind a compact neutron star remnant, but stars this massive may be the progenitors of so-called pair-instability supernovae, explosions so violent that they rip apart the star altogether.

Much of what we can see in the universe is made up of hydrogen. Some heavier elements such as helium were created naturally as the universe cooled following the Big Bang, and others have been created in violent supernova explosions. Under the right conditions, elements combine together forming molecules, and evidence has been found for hundreds of simple molecules in space, including several more complex ones. Now, evidence has been found for a particular form of molecular carbon in a planetary nebula.

In a paper published in Science Express on July 22nd, Jan Cami of the University of Western Ontario and colleagues describe the detection of molecules known as fullerenes in the spectrum of a planetary nebula - the gaseous ejecta left over when a Sun-like star reaches the end of its life and becomes a white dwarf. Prior to the white dwarf phase, the star ejects its outer layers of material in an outflow. Chemical reactions in this outflow turn the material into a variety of molecules and, eventually, dust grains.

The team observed the circumstellar material of Tc 1, a young planetary nebula, using the Infrared Spectrograph (IRS) on the Spitzer Space Telescope. What they found were so-called Diffuse Interstellar Bands in the near-infrared part of the spectrum, at wavelengths identical to those seen in laboratory experiments with fullerenes.

Fullerenes are a peculiar form of carbon resembling a football in structure. Also known as buckyballs, fullerenes are molecules consisting of large numbers of carbon atoms arranged in five or six-sided faces, effectively like the panels which make up the surface of a football.

Fullerenes come in different sizes, but these observations show evidence for forms containing sixty and seventy carbon atoms. In the laboratory, the formation of fullerenes is inhibited by the presence of hydrogen, so the presence of such molecules in this nebula suggests that the environment around Tc 1 is hydrogen deficient. The authors suggest that this unusual environment is a result of the star having ejected its outer hydrogen envelope a few thousand years ago, exposing the underlying helium shell.

These two molecules are the largest so far found in space. They make up a few percent of the total carbon present in the nebula, showing for the first time that if the conditions are right, fullerenes can form efficiently in space.

Most stars in our galaxy orbit around the galactic centre according to well-known laws of motion. The Sun travels in its orbit around the galactic centre at a velocity of approximately 220 km/s. A rare class of fast moving stars, known as hypervelocity stars, move much faster. Only a handful of such stars are known, all discovered since 2005 and thought to have been ejected from the galactic centre at some point in the past. Now, a team of astronomers using the Hubble Space Telescope have found a hypervelocity star with one of the fastest speeds yet, and confirmed that it did in fact originate in the Milky Way's core. The star (known as HE 0437-5439) is travelling through space at 2.5 million kilometres an hour, three times faster than the orbital motion of our own Sun. The team, led by Warren Brown of the Harvard-Smithsonian Centre for Astrophysics in Massachusetts, used Hubble observations to measure the direction of motion of the star and found that its trajectory points directly back to the centre of our galaxy. The star is travelling twice as fast as it would need to in order to escape the gravitational pull of the galaxy, something that does not occur under normal circumstances, so something unusual has to have happened in order to give it such a kick. The team calculated that, based on the star's velocity and current position, it must be 100 million years old if it has indeed been thrown out of the core. The twist is that the star's mass (about nine times that of the Sun) and blue colour imply that it should have reached the end of its life after just 20 million years. In a paper published in the Astrophysical Journal Letters during July, the team suggest that the most likely explanation is that the star was originally part of a triple system which underwent a cosmic game of pool with the black hole at the centre of the galaxy. Their model involves a pair of closely orbiting stars, together with a third star orbiting further out, which came close to the black hole.

The black hole pulled the outer star away from its companions, transferring some momentum to the remaining binary system and increasing its velocity. As they travelled away from the galactic core, the pair continued to evolve as normal, with the more massive star reaching the red giant phase first. As it expanded, the outer layers of the giant star swallowed up its companion and the two stars spiralled together, merging to form the star we now see, an example of the class known as blue stragglers - stars that appear bluer and younger than nearby populations suggest they should. The team are using the same techniques to determine the histories of several other gravitational unbound hypervelocity stars located in the outer regions of the Milky Way.

And finally: On July 23rd,NASA released the highest resolution maps so far of the entire surface of Mars. The Thermal Emission Imaging System, or THEMIS, has been imaging Mars from orbit on board NASA's Mars Odyssey spacecraft which began science operations around the planet in February 2002. The detailed map has been constructed from data obtained over the last eight years by combining nearly twenty one thousand individual images to create a giant mosaic which is available to the public. At full zoom, the smallest surface details are just 100 meters wide. According to Jeffrey Plaut, Odyssey project scientist at JPL, "the map lays the framework for global studies of properties such as the mineral composition and physical nature of the surface materials." While portions of Mars have been mapped at higher resolution, this map provides the most accurate view so far of the entire planet.

The Night Sky

Northern Hemisphere

The bright star Arcturus is in the south-west in the constellation of Boötes. Just below and left is the arclet of stars comprising the Corona Borealis. To the left of that, the four brightest stars of Hercules form the Keystone. Looking with binoculars up the right-hand side of the Keystone, you can see the fine globular cluster M13. Below that is the constellation Ophiuchus, and further down lies Scorpius, containing the bright red star Antares. Scorpius and, to its left, Sagittarius are only visible from more southern latitudes. The direction of Saggitarius is towards the centre of the galaxy. Up and right of the Teapot formation, the Lagoon Nebula is visible. Meanwhile, the imaginary tea from the spout of the teapot falls through the M7 star cluster, above which is the smaller cluster, M6. The Summer Triangle of Cygnus, Lyra and Aquila rises in the south-east, the constellations marked by their respective brightest stars, Deneb, Vega and Altair. Brocchi's Cluster, containing the Coathanger asterism, resides in The dark patch of the sky between Altair and Vega known as the Cygnus Rift. The constellation of Ursa Major, the Great Bear, is overhead. Within this, the middle star of the handle of the Plough can be resolved with binoculars as a binary system consisting of Alcor and Mizar. A telescope reveals that Mizar itself is contains multiple stars. Another multiple star system is Epsilon Lyrae, the Double Double, near Vega. Epsilon1 Lyrae and Epsilon2 Lyrae can be resolved with the naked eye, and each is in fact a double star, the full quadruplet observable through a telescope.

The Planets

Jupiter, like the other naked-eye planets, is now an evening object, rising around 20:30 British Summer Time (BST; one hour ahead of Universal Time) and fairly high in the south-east by the early part of the night. As well as observing Jupiter's four largest moons, the so-called Galilean Satellites, a small telescope allows you to see that the Jupiter's dark South Equatorial Belt is currently missing, while the Great Red Spot is more than usually prominent.

Saturn is very low in the south-west after sunset, in the constellation of Virgo, below and left of Leo. Its relatively dim magnitude of +1 is attributable to the acute angle of its rings to our line of sight, which increases to about 5° by the end of the month.

Mercury appears 3° above the horizon in the west-north-west at twilight, with a magnitude of +0.3, about 20° to the lower right of Venus.

Mars is visible in the south-west in the early evening, near Saturn and between the stars Regulus in Leo and Spica in Virgo. Its present angular size of 4” prohibits the discernment of surface detail without a large telescope.

Venus is at its brightest, shining at magnitude -4.6 in the evening sky. Its angular size increases from 20 to 28” during the month, its effective reflecting area of sunlight decreasing as it approaches Earth. It resembles the first-quarter Moon in shape, approaching its greatest angular separation from the Sun, as seen from Earth, on the 19th.

Highlights

The Perseids, collectively making the year's most dependable meteor shower, shoot across the sky from around the 11th to the 14th. The thin waxing crescent Moon does not obscure the shower after 23:00 BST, leaving dark skies. The best time to see the meteors is 00:30 to 03:00, when their radiant, or point of origin, Perseus, is visible. You can expect a peak of 20 to 30 meteors per hour on the morning of the 13th.

A planetary trio occurs low in the west around the 7th, as Saturn, Mars and Venus appear within 4.7&deg of one another in the sky. The thin crescent Moon also comes within a few degrees on the 12th. Although Saturn moves away during the month, Mars and Venus are 1.9° apart on the 18th.

The Moon's Straight Wall can be seen through a small telescope around the 2nd or the 18th, or a day or two before its third quarter or after its first quarter in any month. It manifests as a bright line reflecting the sunlight early in the month, as the Moon wanes, and as a dark shadow against the Sun later in the month, as the Moon waxes once more.

Uranus can be located through binoculars during August, by using Jupiter as a guide. Two objects of similar brightness lie a few degrees to the right of Jupiter, all three within a single field of view. The leftmost of the two objects is a 6th magnitude star, while the rightmost, showing a slight green-blue tint, is Uranus at magnitude +5.8.

Southern Hemisphere

The constellation Scorpius dominates the sky high overhead in the winter evenings, the Scorpion perpetually chasing Orion, the Hunter. At the heart of Scorpius is the fifteenth-brightest star in the night sky, Antares, literally the 'rival of Mars'. Named for its red colouring, this supergiant lies around 600 light years from us and is intrinsically some 7,000 times brighter than our Sun in visible light. It is so large that it would engulf the Earth if it were placed at the location of the Sun. It is in a binary system with a faint companion star. The globular cluster M4 can be seen nearby using binoculars, and long-exposure photographs reveal bright nebulous patches and dark strands. Scorpius' tail is interpreted by Māori astronomers as Te Matau o Māui, a great hook used by the demigod Māui to raise the North Island of New Zealand from the sea. In this legend, the island was formed from a huge fish, the capture of which caused the hook to fly into the heavens. This leads to the island's name of Te Ika-a-Māui - the fish of Māui. The Southern Island is sometimes called Te Waka a Māui - the canoe of Māui - after the vessel from which the fish was caught. Antares, or Rehua, is the drop of Māui's blood used as bait on the hook. The tip of the hook crosses the Milky Way and ends near to Sagittarius. Here are found various nebulae and star clusters, including M7, which is visible to the naked eye. The nearby Butterfly Cluster, M6, can be viewed through binoculars. Sagittarius, the Archer, follows Scorpius. Within this, the Teapot asterism points towards the centre of the Milky Way, 30,000 light years distant. Our Sun is one of roughly 400 billion stars in the spiral of the Milky Way, and takes around 250 million years to perform one orbit of it. Many bright clusters and nebulae reside in the central region of the galaxy, including the Lagoon and Trifid Nebulae.

The Planets

Mercury, Venus, Mars and Saturn are high in the west, with Mars and Saturn close together but moving apart during the month.

Jupiter reappears in the east during the August nights. Its vanished South Equatorial Belt may return in the coming months, allowing an investigation into its transience.